There is a clear need in engineering for light weight yet wear resistant parts, produced in an economic manner. Typically, materials which are resistant to wear are often inherently brittle, and therefore if used in moving parts which are dynamically loaded (for example in an engine), there is a risk of fracture. One way of overcoming this problem is via the application of a hard wearing exterior coating to a tough and ductile core, for example a ceramic coating on a metal core. The conventional means of providing such wear resistant surface coatings are based on plasmas, e.g. physical vapour deposition (PVD), chemical vapour deposition (CVD), or the like, and thus require expensive equipment, while only depositing a very thin layer, normally in the order of micrometers, which will quickly wear during use. In addition, in coated substrates, severe stresses may build up when the component is subject to heating or cooling, due to the mismatch of thermal expansion co-efficients between the coating and its substrate. This may result in spalling the coating, and delamination of the substrate-coating interface.
Another light yet wear resistant material is a metal matrix composite (MMC). This is a material with a metallic matrix incorporating reinforcing ceramic particles, for example of silicon carbide (SiC). It is however problematic to ensure proper adhesion (wetting) between such particles and the metallic matrix, normally aluminium. In addition, when these materials are melted for casting, the ceramic particles tend to agglomerate or sediment to the bottom of the component. Porosity is a feature of materials thus processed, and is very difficult to avoid. The raw materials are also relatively expensive.
Spray casting is a further method by which fine microstructures may be formed in hypereutectic Al—Si alloys. The process involves atomisation of a stream of molten metal with an inert gas, and deposition onto a moving substrate making the process relatively expensive, and incapable of producing components to a near net shape—only preliminary shapes may be produced, which require subsequent processing to form useful components.
It is therefore an object of the present invention to provide a novel method of producing a functionally gradient component comprising at least one outer layer of a first material having certain physical characteristics, and an inner core of a second material having different physical characteristics, with a gradual change in microstructure between the first and the second material.